Choanocytes

Choanocyte chamber of a Dysidea sponge, showing individual collar cells (choanocytes, with a chemically stained nucleus) lining an oval chamber. The central whip-like hairs (flagella) can be seen in the centre of the chamber.Choanocytes (or 'collar-cells'), are grouped within chambers. They are the main ‘pumping stations’ for sponge survival. These collar cells have a microscopic central hair-like whip (flagellum) that actively beats to create a water current. Surrounding this flagellum is a collar of smaller hairs (cilia) that trap the incoming food particles within in this inflowing water current. These food particles are transported into the base of the collar cell; through the cell wall; and into the body of the sponge (mesohyl) where they are then eaten by amoeba-like cells (archaeocytes). The archaeocytes transport this food around the body. Sponges can actively pump up to 10 times their body volume each hour, making them the most efficient vacuum cleaners of the sea. There are 7,000-18,000 choanocyte chambers per cubic millimeter of sponge, mainly in the upper part of the body, and each chamber may pump approximately 1,200 times its own volume of water per day.

There are exceptions, however. Some sponges living in the deep sea where there are nota lot of nutrients to filter feed, have lost their choanocytes. These species instead use long needle like spicules protruding from their bodies to trap their prey and special cells to digest it, effectively becoming carnivorous.

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